1. Field of the Invention
The present invention relates to a permanent magnet stepping motor, and particularly relates to a permanent magnet stepping motor for use in office automation equipment such as a laser beam printer (hereinafter abbreviated to "LBP") which is required to be low in cost, small in size and high in output.
2. Description of the Related Art
Conventionally, as inexpensive three-phase permanent magnet stepping motors, motors of the inner rotor type in which the number of magnetic poles of each rotor is 6 or more have been much used, and motors in which the number of magnatic poles of each stator is selected to be small are used in order to make the motors inexpensive.
In order to make a permanent magnet stepping motor for use in office automation equipment high in output, it is necessary to make the diameter of its rotor large. In a permanent magnet stepping motor of the inner rotor type, however, there has been a problem in that the diameter of the rotor cannot be made large relatively to the outer diameter of the motor if the outer size of the motor is determined.
In addition, as for the number of phases of the motors, there are motors of the two-phase type, three-phase type, four-phase type and five-phase type. In the motors of the four- or five-phase type, the drive circuit becomes complicated correspondingly to the number of the phases and therefore the motors become high in cost correspondingly.
On the other hand, as the number of phases of the motor increases, the smaller the vibration of the motor during rotation of the motor.
Further, in the case of practical application to an LBP, it is also necessary to reduce vibrations during rotation in addition to the requirements on its low cost and high output. In order to satisfy the requirement both on the low cost and low vibration at the same time, a three-phase motor is better than a two-phase motor in general viewpoint, but there have been no conventional three-phase permanent magnet stepping motors which can satisfy the above conditions.